Soil NO3-δ15N compositions of plant-soil ecosystems on the Chinese Loess Plateau

Abstract:

Soil NO₃-δ¹⁵N values are extremely valuable for understanding nutrient biogeochemical cycling in plant-soil ecosystems. Here, we investigated the δ¹⁵N values of surface bulk soil, soil NO₃ and plants from six restoration grasslands in two different regions (Yunwu Mountain and Bin County) of the Chinese Loess Plateau to study soil NO₃ conversion and transportation in these ecosystems. The soil NO₃-δ¹⁵N values of Yunwu Mountain ranged from -0.1‰ to 1.3‰, and the soil NO₃-δ¹⁵N values of Bin County ranged from 1.2‰ to 11.3‰. In our result, (1) the soil NO₃-δ¹⁵N values increased as plant δ¹⁵N values decreased in study areas. Additionally, the bulk soil δ¹⁵N values were positively correlated with plant δ¹⁵N values. (2) In both study areas, the soil NO₃-δ¹⁵N values were positively correlated with soil water content. In contrast, the bulk soil δ¹⁵N and plant δ¹⁵N values were negatively correlated with increasing soil water content. It is shown that soil NO₃ is easily absorbed by plants in our study areas, which indicates that NO₃ is the primary inorganic N source for grassland plants on the Chinese Loess Plateau. The plants absorbed more ¹⁵N-depleted NO₃ when soil water content increased. Fallen litter assimilated by the soil produced the positive correlation between bulk soil δ¹⁵N values and plant δ¹⁵N values, which may explain how the plant-soil N cycle responds to soil water changes and may explain the negative correlation between bulk soil δ¹⁵N values and mean annual precipitation.